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The present invention relates to variable displacement hydraulic piston pumps, and more particularly, to such pumps which include an adjustable stop assembly for varying the fluid displacement of the pump. A piston pump of the type to which the present invention relates is illustrated and described in U.S. Pat. No. 5,782,160, now assigned to the assignee of the present invention, and incorporated herein by reference.
The variable displacement piston pump of the above-incorporated patent includes an adjustable maximum displacement stop assembly for limiting the maximum angle of the swashplate (yoke) to something less than the maximum angle possible for the particular pump design. As is well know to those skilled in the art, for a given input speed to the pump, the rate of fluid flow provided by the pump is proportional to the angle of displacement of the swashplate. Therefore, many piston pumps, especially those intended for industrial uses (as opposed to mobile applications) are designed for a maximum displacement (maximum flow) and then are xe2x80x9csetxe2x80x9d or adjusted for a particular use by having the swashplate positioned at a particular angle, less than the maximum, which angle (effectively, a new xe2x80x9cmaximumxe2x80x9d displacement) will provide the desired flow rate. Once the desired swashplate displacement is set, the adjustable stop assembly is prevented from further movement by some suitable locking means.
In the adjustable stop assembly of the above-incorporated patent, there is a control piston which is linked to the yoke, the control piston defining an internal, hexagonal cavity in which is disposed a hexagonal stop member. The stop member is in threaded engagement with an adjusting screw, such that rotation of the adjusting screw results in linear movement of the hexagonal stop, and therefore, linear movement of the control piston also. It is the linear movement of the control piston which directly changes the displacement angle of the swashplate.
It should be noted that in the adjustable maximum displacement stop assembly of the cited patent, the control piston, the hexagonal stop, and the adjusting screw are all coaxial with each other.
The prior art adjustable stop assembly has been generally satisfactory in performance, i.e., in achieving and maintaining the desired displacement angle of the swashplate. However, the inherent requirement for certain parts of the assembly to be non-circular (preferably, hexagonal) adds substantially to the cost and difficulty of manufacture of at least two components of the assembly, in this case, the control piston and the stop. In addition, the prior art assembly required a mechanical connection between the swashplate and the control piston, partly for the purpose of preventing rotation of the control piston. In the prior art assembly, if the control piston were permitted to rotate, there would be no linear movement of the control piston. Typically, the mechanical connection between the swashplate and the control piston is a member referred to as a xe2x80x9cchain linkxe2x80x9d, which is not an especially difficult or complicated part, but on some applications of the pump, does represent one additional part which serves no purpose, other than to prevent rotation of the control piston.
Accordingly, it is an object of the present invention to provide an improved variable displacement hydraulic piston pump, and an improved adjustable maximum displacement stop assembly therefor which overcomes the above-described disadvantages of the prior art.
It is a more specific object of the present invention to provide such an improved adjustable stop assembly which eliminates the need for expensive and difficult-to-manufacture non-circular parts.
It is another object of the present invention to provide such an improved adjustable stop assembly in which is not necessary to provide means for preventing rotation of the control piston.
The above and other objects of the invention are accomplished by the provision of an improved adjustable stop assembly for limiting maximum yoke angle of a hydraulic pump having a fluid displacement proportional to the angle of the yoke, the adjustable stop assembly including a control piston for inclining the yoke to define a desired yoke angle. A stop member defines a set of internal threads and is disposed within a cavity defined by the adjustable stop assembly. The assembly also includes an adjusting screw disposed for rotational motion, comprising means operable to retain the adjusting screw reciprocally stationary relative to the adjustable stop assembly. The adjusting screw further comprises a first set of external threads for engagement with the internal threads of the stop member whereby rotation of the adjusting screw causes axial travel of the stop member thereby displacing the control piston.
The improved adjustable stop assembly is characterized by the cavity defining a generally cylindrical internal surface, and further defining a first axis about which the internal surface is concentric. The set of internal threads defined by the stop member defines a second axis, the first and second axes being transversely offset.